- Move Oscar app configs into Django app config classes
- Rename Oscar/Django apps' app config module
- Remove "oscar.get_core_apps()" and its app overriding
- Remove "oscar.OSCAR_CORE_APPS"
- Port app forking code to work with Django app registry
- Port class loading code to use app configs to get app properties

Django 2.1 works fine - the only issue was that more passwords
have been added to the CommonPasswordValidator which caused some tests
to fail. I've removed that validator from the test settings.

See #1674.

The procedural views were deprecated in Django 1.11 and will be removed in Django 2.1.

If a phone_number field is required, and empty, then PhoneNumberMixin.clean() fails with a key error because the field does not exist in cleaned_data.

This patch fixes the clean() method to only attempt to clean a field if it exists in cleaned_data. If it doesn't then allow Django to raise a validation error for a missing required field.

Also added more comprehensive tests for the validation logic in the mixin.

Support for the old-style attributes was dropped in django-extra-views 0.11.

Also pin the version as the package does not use semantic versioning.

Fixes #2688.

Revert changes in #1913 and #2294 that required a stock record to have price_excl_tax set in order to report the product as available.

Add separate checks to the basket form/add logic that checks at the time of
adding a product to the basket whether a price exists, and report an error if it doesn't.

Fixes #2664.

Also forward-port 1.5.3 release notes and clean up release notes for 1.6

This can be enabled by specifying a `OSCAR_DEPRECATED_ORDER_VERIFY_KEY` setting, which is not set by default.

This allows projects to continue validating old order verification hashes while still changing the `SECRET_KEY`.

Update other deprecation notices to use RemovedInDjango20.